I'm on my third amp build, but have a problem that's got me stumped. I get this:

at C18/R32 when I get the Variac up to 80VAC. It's not constant, but a pulse that only lasts half a second, repeats about every two seconds. There is a corresponding jump in amperage draw and needle deflection. If I turn the Variac up any more it goes nuts. Haven't blown a fuse. Rechecked all components and solder joints in both feedback loops. It goes away if the 12AX7 is removed or if all of the KT120's are removed. Something is oscillating, but I can't figure out what. I tries different 12AX7 and 12AT7 tubes. All tubes test good. I've even tried just one KT120, but it's still oscillating no matter which position it is in. Any thoughts?

Try swapping the signal wires that go to the output transformer from the KT-120 outputs on the left channel. The schematic does not show any particular polarity of them but in order for the feedback to work correctly they cannot be 'backwards'. I had this problem with my amp at CF, I didn't see it on a scope but when trying to run it at Cabin 2 it was oscillating. Compared it to the working amp that was the source of the trouble.

This is similar to what BDR is suggesting, but it's not in the feedback, it's on the supply side of the output transformer.

Yes, thank you Mr. UncleMeat, that's better said. The whole circuit is a closed loop when you close it with feedback. The point is it has to be negative, not positive, to avoid jumping immediately into wild oscillation. Without any documents to go on in front of me, I was having trouble visualizing the specifics.

Over the holidays my dear old Mother said, "Benny, you need to spend more time with your toys, the things you used to enjoy so much". And she's right of course. So hear I am, trying to follow her advice and get myself back into the audio groove again.

On a couple of occasions I have turned my amp on and it starts to buzz as the bias comes up. I think this has happened when it's been turned off for a long while and the main supply caps are discharged and the amp is cold. This problem goes away if I turn the amp back off, wait 30 seconds and turn it back on. It's something to do with the bias charging up and the tubes stabilizing. I'm thinking if the tubes are not warm/stable before the bias voltage comes up it will buzz and form a feedback loop.

This has only happened a 2-3 times, and it's dead quiet when it's stable. I think it's a slight design flaw in the way the amp warms up since there is no muting circuit. It may or may not be related to the problem that Kevin posted; but I thought I'd post here since it's potentially a common issue/problem.

When a tube is powered up, you actually desire the bias to be there before the B+ if anything. The bias controls the tubes gain which is why logically it needs to be there first. How the power supply in that unit comes up, I don't know.

In the typically supply you have filaments and then the HV rail. In some you have a third winding for the bias or derive it somehow otherwise. The first off the HV supply is the B+ and the others, if there is, follow. Bias being negative has to be designed a bit different that the other HV supplies. Less bias (closer to zero) means more gain. Further negative is less multiplication by the tube so it needs to charge up with enough time to get there. As it all comes up to the tube, the B+ versus the bias could be a concern.

I don't know any specifics, so consider this above thinking to be just me thinking out loud please.

Yes, you are correct Ben. My reverence to the bias was as it's seen on the meter; which is in mA. When the amp is turned on the meter reads 0mA and it slowly rises to 80-100mA, which is the idle bias -current- when it's warm/running at idle. This doesn't say anything about what the bias voltage is when it's switched on it may be -47V (or higher) when it first turned on. I think the bias, and the B+ are probably present and close to their 'warm' voltages, and what we see when the bias current slowly rises is the cathode being warmed by the filaments.

The problem I'm seeing when it's turned on as a cold amp is that an oscillation is starting and creating some form of positive feedback. I would have liked to have a standby switch, so I'm considering putting in a 3-position switch instead of the stock on/off switch. Center position could turn the amp on but have the B+ to the power tubes turned off.